High voltage slewing of penetration tube gun

ABSTRACT

MAINTAINED AT A CONSTANT HIGH VOLTAGE POTENTIAL, WHEREAS THE SHIELD AND THE GUN ARE SLEWED MANY KILOVOLTS BETWEEN HIGH VOLTAGES SO AS TO PROVIDE VARIABLE TO PENETRATION OF THE CRT BEAM IN ORDER TO EXCITE SELECTED PHOSPHORS.   THE ELECTRON GUN OF A MULTIPLE PENETRATION TYPE CATHODE RAY TUBE IS COMPLETELY SURROUNDED BY A SHIELD WHICH HOUSES CIRCUITRY FOR OPERATING THE GUN WITH POTENTIALS REFERENCED TO THE SHIELD. THE CIRCUITRY IS COUPLED THROUGH HIGH VOLTAGE ISOLATION TO EXTERNAL CIRCUITRY SUCH AS POWER SOURCES AND A SOURCE OF VIDEO. THE ANODE OF THE CRT IS

R. F. WEST Marh 13, 1973 HIGH VOLTAGE SLEWING OF PENETRATION TUHH GUNFiled NOV. l0. 1971 United States atent 3,720,781 HIGH VOLTAGE SLEWING FPENETRATION TUBE GUN Roger E. `West, Weston, Conn., assignor to UnitedAircraft Corporation, East Hartford, Conn. Filed Nov. 10, 1971, Ser. No.197,467 Int. Ci. H04n 9/22; H01j 1/52; H04b 1/08 U.S. Cl. 178-5.4 PE 6Claims ABSTRACT 0F THE DISCLOSURE The electron gun of a multiplepenetration type cathode ray tube is completely surrounded by a shieldwhich houses circuitry for operating the gun with potentials referencedto the shield. T-he circuitry is coupled through high voltage isolationto external circuitry such as power sources and a source of video. Theanode of the CRT is maintained at a constant high voltage potential,whereas the shield and the gun are slewed many kilovolts between highvoltages so as to provide variable to penetration of the CRT beam inorder to excite selected phosphors.

BACKGROUND OF THE INVENTION Field of invention This invention relates toimprovements in arrangements of high voltage in multiple phosphorpenetration cathode ray tube circuitry.

DESCRIPTION OF THE PRIOR ART As is known in the art, a cathode ray tubecomprises an evacuated glass bulb, necked down at one end wherein anelectron beam gun is situated. Various focusing electrodes, grids andsweep generating coils or plates and other apparatus are also locatedalong the necked down portion. The working element of the electron beamgun is the cathode which is nominally held at a low potential. Toaccelerate the electrons rapidly into a beam which has a substantiallyconstant velocity throughout most of the length of the tube as theelectrons flow toward the screen of the tube, a conductive coating isutilized throughout substantially the entire internal surface of thetube, except immediately adjacent to t-he gun. In addition, a thinaluminum layer (or the equivalent) covers the entire inner surface ofthe screen (the front face of the tube) and a high voltage (5 kv. to 20kv. is typical) is applied to this inner conductive layer, and to theconductive coating, which operates to accelerate the electrons to theproper velocity to impact the particles of the phosphor in a fashion tocreate the light image as desired. The high voltage on this coating,however, creates problems with metal surfaces and circuitry external tothe tube; that is to say the coating on the inside of the tube forms acapacitor with any conductive element adjacent the outside of the tube.To isolate the high voltage in the tube from circuitry surrounding thetube, it isr very common to employ a conductive coating on the outsideof the tube which is typically a carbon wash of some sort. This outercoating is grounded so as to provide the desired isolation. However, theouter and inner coatings, separated by a dielectric (the glass of thetube Wall), form a capacitor.

Multiple phosphor penetration type `cathode ray tubes have additionalproblems. A multiple phospor penetration type tube is one which eitherhas composite phosphors in which an inner phosphor is one color (greenor blue), completely surrounded by a different phosphor (such as red),all of these particles together forming the phosphor surface on thescreen. If the electron beam impacts the screen with a very highvelocity, the inner phosphor will be excited and the light generatedwill be the color of the inner phosphor; if however the beam impacts thephosphor with a lesser velocity, then only the outer layer is excitedand so the other color (such as red) will emanate. Another type ofmultiple phosphor penetration type tube employs two separate layers ofphosphor separated yby an energy sensitive barrier layer. Otherpenetration CRTs,i similar to the multi-color penetration CRTs, mayemploy phosphors of different persistence or other characteristicsrather than only of different colors. Although general knowledge islimited as to the method of manufacture of tubes of this type, of highquality, with a high contrast between the effect of the selectablephosphors as a function of control of the voltage which accelerates thebeam, tubes of the type described hereinbefore are readily available incommerce from companies such as RCA, Sylvania, and others.

The key to operation of a multiple phosphor penetration type fCRT is toprovide a beam of the correct energy on a point by point basis as apicture is being developed on the screen of the CRT. In order to alterthe intensity of the beam from point to point as necessary to excite thedesired phosphor in the generation of the picture, it -becomes necessaryto switch the high voltage at video rates. Thus, once the high voltagestarts switching at high frequency, the entire system acts much the sameas an antenna, creating strong fields about the tube. For instance,switching from 6 to 12,000 volts at video rates, comprising a squarewave, provides significant frequency components as high, perhaps, a 50kHz. If one were to approach the front of the screen one would haveextreme discomforture and perhaps even feel a signicant shock as aresult of this high voltage field. In order to overcome this problem inpenetration type tubes, an RF shield is placed in front of the screen,which is grounded so as to isolate persons and objects in the vicinityof the front of the screen from the RF eld generated by switching of thehigh voltage. This in turn provides another significant capacitancebetween ground and the high voltage acceleration layer within the tube.

The total capacitance of the high voltage acceleration layer or anodeinside of the tube to ground (represented by the shielding coating onthe outside of the tube and the RF shield at the front of the tube)renders it difficult to change the high voltage by large amounts at arapid rate. For instance, to change from an acceleration voltage on theorder of 6,000 volts to an acceleration voltage on the order of 14,000volts (which may obtain in a typical large screen application), mayrequire driving as much as several thousand pico farads, which could inturn require as much as two or three amperes. This can be seen torequire a power supply capable of delivering on the order of l0 to 20thousand watts peak.

Assuming that one has a suitable power supply to handle theaforementioned diiculties, there is nonetheless an additional associatedproblem. Because of the difliculties of connecting the anode coatinginside of the tube with a power supply, through the tube wall, anodevoltages are typically applied at one electrode only. However, in alarge screen CRT (which may be as much as 24 inches in diameter), it isobvious that all of the capacitance described hereinbefore isdistributed about a large area of tube surface. Thus the voltage maychange more quickly at points immediately adjacent the anode terminalthan would occur on the opposite sides of the tube, which are somedistance away, due to the resistance of the conductive coating. The tubemust therefore be blanked out once a voltage change is initiated untilsteady state conditions can be established with an equipotential layerthroughout the screen of the tube. This slows down the rate at which thevideo can be changed in the tube; other- Wise, defocusing and errors indeection and other picture degradation will result.

SUMMARY OF INVENTION The object of the present invention is to provideimprovements in multiple phosphor penetration CRT systems, andparticularly in the high voltage arrangements therein.

According to the present invention, the anode of a multiple phosphorpenetration cathode ray tube is held at a constant potential, and thegun of the cathode ray tube, including the immediate circuitry foroperating it, are all slewed upward and downward by the desired voltagedifferential.

In further accord with the present invention, the gun and its associatedequipment are located within a Faraday shield, whereby the slewing ofthe high voltage has no effect on the circuitry within the Faradayshield. In still further accord with the present invention, essentiallyonly two connections are made between the circuitry within the Faradayshield and the outside world; these comprise an alternating currentsource of power which may conveniently be supplied thereto by a twistedpair fed through a high voltage isolation transformer, and videosuperimposed on an RF carrier supplied over a coaxial line through ahigh voltage isolation transformer.

The present invention provides significant reduction in the capacitancewhich has to be driven in a high voltage voltage-switching multiplephosphor penetration cathode ray tube. 'In addition, the presentinvention avoids the necessity of an RF shield since the high voltage atthe screen remains constant. The present invention, by keeping thescreen at the same potential at all times, completely eliminates thenecessity of establishing steady state conditions each time that thehigh voltage is switched in order to avoid focus and deflectionambiguities and the resulting degradation of the picture quality.

The present invention is readily implementable using standard technologywithout signicant penalties in cost, weight or size.

Other objects, features and advantages of the present invention willbecome more apparent in the light of the following detailed descriptionof a preferred embodiment thereof, as illustrated in the accompanyingdrawing.

BRIEF DESCRIPTION OF THE DRAWING The sole figure herein comprises asimplified, illustrative schematic block diagram of a preferredembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing, atypical embodiment of the present invention is shown as it may beimplemented in a video display unit. A cathode ray tube 1 is providedwith a fixed high voltage Supply 2 which is connected to an anodeelectrode 3 on the CRT to provide the acceleration voltage to the anodeof the CRT. The CRT 1 may be provided with an external conductivecoating 4 which is grounded to isolate structure external of the tube 1from the high voltage on the anode, as is known in the art. The CRT isalso provided with a focus coil 5 which is driven by suitable focuscircuitry 6 which may be of the type well known in the art and which isnot described further herein. A deflection coil 7 is driven by a symbolgenerator 8 which may in turn receive signals from apparatus, such as acomputer, a radar system or other apparatus, the data output of which isto be displayed. The symbol generator 8 also provides video to amodulator 9l which modulates the video on an RF carrier provided theretofrom an RF carrier source 10.

In accordance with the invention, a high voltage switching power supply12 is controlled by the symbol generator 8 so as to provide a swing inhigh Voltage applied to the gun 14 of the CRT 1. The high voltage supply12 is connected to a Faraday shield structure 16, which completelysurrounds the gun 14 and circuitry utilized to operate the gun 14. TheFaraday shield 16 may take the form of a hollow aluminum cylinder, orother shapes of other conductive materials as may be desired in anygiven utilization of the present invention. As shown in the figure, theFaraday shield 16 is supported by an insulating disc 15 which slips overthe neck of the tube. Additional support may be provided by similarinsulating discs located midway within the shield (but eliminatedherefrom for simplicity) which may conveniently have the CRT tube socket(not shown) fastened thereto, whereby the Shield 16 will derive supportfrom the base (not shown) of the tube; however, the Faraday shield 16may be supported by external structure or in any other way foundsuitable.

Located within the Faraday shield 16 is a detector and amplifier 18 of acommon type which is utilized to drive the control grid 20 of the CRT.Also, a suitable power supply for the video amplifier is included withinthe Faraday shield 16; this may typically comprise a 80 Volts supply 22.A SOO-volt supply 24 is provided to maintain the potential of the screengrid 26 of the CRT. Power` for the power supplies 22, 24 is provided bya twisted pair of leads 2S which is connected externally of the Faradayshield 16 to a medium frequency power supply 30, which may typicallysupply 6.3 volts (to match the heater voltage of the CRT) at about 50kHz. The power supply 30 iS coupled to the twisted pair 2S by a highvoltage isolating transformer 32 which may comprise a ferrite core withsuitable insulation and a proper assemblage of windings in accordancewith well known teachings of the art. Within the Faraday shield 16, onewire 34 of the twisted pair comprises ground and is connected to theFaraday shield 16. One side of the heater is similarly grounded to theshield 16, and the other side of the heater is connected directly to theother wire 38 of the twisted pair 28 along with the power supplies 22,24. The cathode 46 of the CRT is connected directly to the shield 16 sothat it is slewed between upper and lower DC voltages applied thereto bythe switching high voltage supply 12. The video detector and amplifier18 derives the video input from a coaxial line 42 which is coupledthrough a single turn to a high voltage isolating transformer which maytypically comprise an iron dust core (or it may be an air core) which iscoupled to a primary 46 driven by the modulator 9. The transformer 43may be designed in any desired fashion so as to satisfy the requirementsof the carrier (which may typically be mHz.) provided by the source 10.

In operation, in accordance with the color of a given symbol to bewritten on the face of the CRT 1, the symbol generator 8 causes the highvoltage switching supply 12 to change from one voltage to another sothat the total acceleration voltage between the cathode 40 and the anode3 of the CRT changes from one voltage to another. As an example, thefixed high voltage supply 2 might be at a potential of positive 9 kv.and, in a two-level penetration type application, the switching highvoltage supply 12 may typically provide minus 4 kv. and plus 4 kv. Thusthe total acceleration voltage would vary from 5 kv. to 13 kv. However,depending on the type of CRT being driven, more than two voltage levelsmay be supplied by the switching high voltage power supply 12; forinstance, in a tube capable of generating four different colors, fourdifferent voltages may be provided by the switching high voltage supply12.

The use of a medium frequency power supply to introduce power to thecircuitry within the Faraday shield 16 is convenient since the size,weight and cost of the transformer can be lower at a higher frequencythan for standard power frequencies. Of course, other frequencies couldbe utilized if desired in any given implementation of the presentinvention.

It should be appreciated that the magnetic coupling provided by the highvoltage isolation transformer 43 in coupling the video from the outsideworld into the Faraday shield 16 is essential in order to isolate thehigh voltage DC of the Faraday shield 16 from the circuitry outsidethereof, In contrast, if one attempted to use capacitive coupling forthe video, the video signal would be superimposed on the high voltage,and it would be impossible to isolate the circuitry within the Faradayshield 16 from the high voltage. The shield 16 should be sucient toprovide substantially complete RF shield of the components 14, 18, 22and 24 from the outside world, so that they can function together intheir own world which is many volts elevated from the surroundingstructure outside the shield 16.

Thus, although the invention has been shown and described with respectto a preferred embodiment thereof, it should be understood by thoseskilled in the art that the foregoing and various other changes andomissions in the form and detail thereof may be made therein withoutdeparting from the spirit and the scope of the invention.

Having thus described typical embodiments of rny invention, that which Iclaim as new and desire to secure by Letters Patent of the United Statesis:

1. A multiple penetration cathode ray tube apparatus adapted to providesignificant rapid alteration of the cathode ray beam acceleratingvoltage, comprising:

a multiple penetration cathode ray tube having an anode and an electrongun including a cathode;

a Faraday shield surrounding said electron gun sufliciently to providesubstantially complete RF shielding thereof;

high voltage isolation means;

external circuit means disposed outside of said shield for supplyingpower and video signals for the operation of said gun;

shielded circuit means disposed within said shield, said shieldproviding substantially complete RF shielding of said shielded circuitmeans, said shielded circuit means providing operating potentials andsignals referenced to said shield for operating said electron gun inresponse Ato signals applied thereto by said external circuit meansthrough said high voltage isolation means;

a fixed high voltage supply connected to the anode of said cathode raytube; and

a switching high voltage supply capable of providing at least twodifferent voltages connected at least to said shield and to saidcathode.

2. Apparatus according to claim 1 wherein:

said external circuit means includes video means providing a videosignal modulated on an RF carrier;

said shielded circuit means includes a video detector and ampliiierdispos-ed in said shield and operating with the potential of said shieldas a reference potential; and

said isolation means includes'a high -voltage isolation RF transformerhaving a primary coupled to said video means and a single turn secondarycoupled through the central conductor of a coaxial cab-le to said videodetector and amplifier, the outer conductor of said coaxial cable beingconnected to said shield.

3. Apparatus according to claim 2 wherein:

said external circuit means includes a medium frequency power source;and

said isolation means includes a high voltage isolation transformerhaving a primary coupled to said medium frequency power source, andconnection means comprising a twisted pair connected to a secondary ofsaid transformer and applying the voltage therefrom between said shieldand said shielded circuit means.

4. Apparatus according to claim 1 wherein:

said external circuit means includes a medium frequency power source;and

said isolation means includes a high voltage isolation transformerhaving a primary coupled to said medium frequency power source, andconnection means comprising a twisted pair connected to a secondary ofsaid transformer and applying the voltage therefrom between said shieldand said shielded circuit means.

5. Apparatus according to claim 4 wherein said electron gun includes ascreen grid and said shielded circuit means further includes a powersupply for said screen grid powered by said twisted pair.

6. Apparatus according to claim 4 wherein said electron gun includes acathode heater element connected to said twisted pair.

References Cited UNITED STATES PATENTS 10/196'8 Alcala et al. l78-7.8 5/1969 Schwartz 315-8

